@striatum What version of Vivaldi are you using? Also, are you using a Snapshot build or Stable release and did you ever switch Vivaldi release streams on your system?

One other thing that you can do is to make sure that your current installation has not been damaged or corrupted. All official releases on macOS are signed and you can verify its integrity with the following Terminal command:

You can try moving your current profile diectory away (e.g. to Documents or somewhere else) and install V without a profile present. Maybe this does make a difference? Again, afterwards you can try to copy back your old profile (I suggest copying it instead of moving; this way you still have a copy if something goes wrong).

The path you'd need to move is
/Users/<yourname>/Library/Application Support/Vivaldi
The Library folder is hidden by default, enable hidden files in Finder before if you need to.

Vivaldi Snapshot (current as soon as possible) on MacOS Sierra and High Sierra. Enough RAM.

@morg42 The steps to resolve this all depend on how or why the update is failing.

Is the updater not able to download the new version?

Did the update "succeed" but Vivaldi crashes when you launch it?

In the past, some users have even tried to have both a Stable version of Vivaldi and a Snapshot installed on macOS at the same time. This is not a supported configuration and can also REALLY mess up the updater.

Download Vivaldi from the official site: https://vivaldi.com/
Trash your current Vivaldi.app and drag the new one into Applications. And probably be more careful with your custom mods next time, could be you messed something up.